Device for Treating Nitrogen Oxides of Motor Vehicle Exhaust Gases

ABSTRACT

A propulsion system, for example for a motor vehicle, that includes an internal combustion engine, a catalyst for reducing nitrogen oxides NOx contained in the exhaust faces of the internal combustion engine, a fuel injector arranged upstream of the catalyst, and a mechanism that determines quantity of the nitrogen oxides NOx received from the internal combustion engine. The propulsion system also includes a mechanism that determines a fuel quantity required for attaining a proportion lower or substantially equal to a stoichiometric proportion with respect to the nitrogen oxide NOx quantity, and a mechanism that triggers injection of the required quantity when the ratio therebetween and the nitrogen oxide NOx quantity is less than the triggering threshold.

The invention relates to the field of propulsion systems, in particular for motor vehicles. The invention relates it particular to the field of devices and methods for the catalytic treatment of nitrogen oxides (NOx) in the exhausts gases of internal combustion engines.

In a manner known per se, nitrogen oxides cause respiratory infections and allergies, and play an influential role in the formation of smoke and acid rain. Most countries have imposed standards limiting the emission of nitrogen oxides from vehicles. These standards generally specify a different threshold for privately owned vehicles, for utility vehicles, and for industrial engines. In fact, each of these categories covers a wide variety of engines. For example, privately owned vehicles include vehicles with gasoline engines and vehicles with diesel engines. They include small urban vehicles and four-wheel drive vehicles. To obtain the same pollutant emission threshold for a high power engine requires more intensive treatment of the nitrogen oxides NOx than to obtain the same pollutant emission threshold for a small engine. Moreover, the economic pressure to propose inexpensive devices for treating nitrogen oxides is more severe for small urban vehicles than for luxury vehicles.

Several technologies have been developed for reducing nitrogen oxide NOx emissions. A distinction is made in particular between passive methods using catalysts on which the exhaust gases are passed, and active methods employing complex processes, such as electrochemical catalysis or photocatalytic approaches. Passive catalytic methods are routinely used for gasoline vehicles. A three-way catalyst is used to simultaneously treat oxides of nitrogen, of carbon, and unburnt hydrocarbons. The exhaust gases from the gasoline engine combustion chambers are generally rich in reducing elements, such as hydrocarbons and carbon monoxide, so that by using a three-way catalyst, the nitrogen oxides (NOx) emitted are reduced to nitrogen gas. These three-way catalysts have the advantage of being inexpensive.

Diesel engines have combustion chambers fed with a mixture containing excess air. The exhaust gases from the engine are therefore poor in reducing agents. The use of the three-way catalyst is unfeasible, because the reducing efficiency of these catalysts rapidly decreases in the presence of oxygen.

NOx traps have been proposed for treating nitrogen oxides in the case of exhaust gases poor in reducing agents. Such NOx traps are described in particular in patent applications EP 0 573 672 A1 (Toyota) and EP 1 079 084 A2 (Toyota). During the normal running of the vehicle, the exhaust gases pass through the NOx trap where the catalysts generally comprise alkaline or alkaline-earth elements and storage compounds. These catalysts promote the oxidation of the nitrogen monoxide NO to nitrogen dioxide NO₂ and the conversion of the nitrogen dioxide NO₂ to nitrates which are deposited on a monolith. During the normal engine operating phase, the NOx are absorbed by the NOx trap. Periodically, the stored Nox are desorbed. During this desorption phase, the proportion of exhaust gas reducing agents is higher than the stoichiometric proportion (mixture called rich). The control of such a NOx treatment is a complex process, in which the desorption phase must be detected and the quantity of hydrocarbons injected must be increased. The storage components of the NOx trap are in particular especially sensitive to sulfur. In parallel with the NOx storage process, it was necessary to develop desulfurization steps at regular intervals.

Patent application WO 02/31325 (Corning Inc.) describes a method for reducing nitrogen oxides in the exhaust gases of diesel engines. This method consists in injecting fuel continuously into the exhaust gases and in passing the mixture of fuel and exhaust gases through a catalyst to reduce the NOx in the exhaust gases to nitrogen gas. This process has the drawback of causing additional fuel consumption.

Patent application US 2004/0083722 (Ford) describes an improved NOx conversion by the use of a catalyst in which the quantity of fuel injected upstream of the catalyst is corrected with regard to a quantity injected when the engine is in steady state running conditions to take account of the engine acceleration and deceleration phases. The catalysts mentioned are of the ALNC or SCR type, which continuously reduce the NOx emissions by an active injection of reducing agents (hydrocarbons or urea). Despite the improvement, the proportion injected in steady state conditions in the example described is 10 times more of reducing agent than of NOx. Despite an improvement, such a system consumes a considerable amount of fuel.

The invention proposes a propulsion system, in particular for a motor vehicle, and a method for treating the exhaust gases of the propulsion system, which remedy these drawbacks and, in particular, which propose an inexpensive treatment of the nitrogen oxides NOx, reducing the extra fuel consumption.

According to an embodiment of the invention, the propulsion system, in particular for a motor vehicle, comprising an internal combustion engine, a catalyst for reducing nitrogen oxides NOx contained in the exhaust gases of the engine, an additional fuel injector arranged upstream of the catalyst and means for determining the quantity of nitrogen oxides NOx issuing from the engine, means for determining the quantity of fuel required for attaining a proportion lower than or substantially equal to the stoichiometric proportion with regard to the quantity of nitrogen oxides Nox, and means for triggering the injection of said required quantity when the ratio of said required quantity to the quantity of nitrogen oxides NOx is lower than a triggering threshold.

It is understandable that such a propulsion system, comprising a NOx reduction catalyst traversed by a substantially stoichiometric mixture, can make use of an inexpensive catalyst, for example of the three-way catalyst type known for gasoline vehicles. Moreover, the additional fuel consumption is eliminated as long as the ratio of the quantity to be injected to the quantity of NOx is lower than a triggering threshold. In other words, the additional injection is stopped when the quantity of NOx present in the exhaust gases has not reached the limit authorized, for example, by the standards. In the case of small urban vehicles, this limit is generally only exceeded during certain running phases of the vehicle, such as a cold engine, a phase of high acceleration or a high load of the engine. Thanks to the means for adjusting the quantity of fuel injected to obtain the stoichiometric proportion, this quantity of fuel is limited. In fact, the phases of engine operation with high NOx emission also correspond to phases in which the exhaust gases are richer than in steady state conditions, in unburnt hydrocarbons and in reducing agents. The quantity of diesel required for injection to reach stoichiometry is commensurately lower.

Advantageously, the exhaust gases upstream of the additional injector are poor in reducing agents. The engine may be of the diesel type and the fuel injected by the additional injector can be diesel fuel.

According to another embodiment, the means for determining the quantity of NOx comprises a computer suitable for receiving data on the engine speed and on the injection into the engine combustion chambers and for comparing these data with stored data. Advantageously, this means comprises, downstream of the engine and upstream of the additional injector, a device for measuring the fuel-air ratio of the exhaust gases.

According to another embodiment, the propulsion system comprises a particulate filter upstream or downstream of the catalyst or incorporating the catalyst. The additional injector may be placed upstream of the particulate filter and may be actuated to regenerate the particulate filter.

Advantageously, the propulsion system comprises a second device for measuring the concentration of reducing agents downstream of the additional injector and upstream of the catalyst. It may comprise a temperature probe upstream or downstream of the particulate filter, and/or a pressure probe upstream or downstream of the particulate filter.

According to another embodiment, the invention the propulsion method, in particular of a motor vehicle, comprises a step in which the exhaust gases from an internal combustion engine are treated by injecting hydrocarbons into the exhaust line upstream of a NOx reducing catalyst, the quantity of NOx present in the exhaust gases upstream of the injector is determined, the hydrocarbon injection is triggered when a NOx quantity threshold is reached, and the quantity injected is adjusted so that the quantity of reducing agents is lower than or substantially equal to the stoichiometric proportion with regard to the quantity of NOx present in the exhaust gases.

Other features and advantages of the invention will appear on a reading of the detailed description of a number of nonlimiting exemplary embodiments illustrated by the appended drawing, in which the single FIGURE shows a schematic representation of the members of a propulsion system according to the invention.

As shown in the FIGURE, the motor vehicle propulsion system comprises an internal combustion engine 1 of the diesel type and an exhaust line 2 comprising a first section 2 a immediately downstream of the internal combustion engine 1 and a second section 2 b immediately upstream of the first section 2 a.

The FIGURE shows four embodiments which differ from one another in their second section 2 b of the exhaust line 2, as indicated schematically by the dotted line.

In the first embodiment, the first section 2 a of the exhaust line 2 comprises a lambda fuel-air ratio probe 3, or oxygen probe, sensitive to the proportion of oxygen in the exhaust gases from the internal combustion engine 1. An additional injector 4 is inserted into the first section 2 a downstream of the fuel-air ratio probe 3. The second section 2 b of the exhaust line 2 of the first embodiment comprises a catalyst 5. The propulsion system also comprises a computer 6.

The operation of this first embodiment is now described. The diesel internal combustion engine is equipped with a crankshaft speed sensor 7 not shown, a sensor 8 for sensing the injection conditions of the main injections of the engine 1. This sensor may, for example, be an accelerator position sensor or a sensor of the control circuits of the main injectors. Optionally, the engine is equipped with a combustion chamber temperature sensor. The data issuing from these sensors are sent to the computer 6. Similarly, data concerning the type of fuel being used can be entered and sent to the computer 6. A complete mapping of the various engine operating speeds as a function of the characteristics of the fuel being used, is stored in the computer 6. This enables the computer to determine the engine speed and, in real time, to determine the proportion of nitrogen oxides escaping from the combustion chambers of the engine 1 and the quantity of unburnt hydrocarbons or reducing components escaping from the engine combustion chambers. The computer 6 can then determine the quantity of reducing elements required to be injected by the additional injector 4 so that the gas mixture thereby modified has a stoichiometric proportion between the quantity of oxidizing elements and the quantity of reducing elements.

When the computer 6 actuates the additional injector 4, and triggers the injection of said required quantity, the mixture passing through the catalyst 5 has the desired stoichiometric proportion. The conditions for which the injection of diesel into the exhaust line is triggered (quantity of NOx emitted by the engine and fuel-air ratio of the gas mixture in the combustion chamber), can be modified to adapt, for example to changes in the legislation in force. It corresponds to the proportion of NOx that will be reduced by the action of the catalyst 5 before escaping from the exhaust line 2. The desired stoichiometric proportion may also correspond to a maximum quantity of NOx which will not be reduced by the catalyst 5. When the catalyst 6 does not actuate the additional injector 4, all the NOx issuing from the internal combustion engine 1 passes through the catalyst 5, and the latter has a substantially lower reducing efficiency due to the low content of reducing agent in the mixture.

In an alternative, the desired stoichiometric proportion is equal to one, so that all the NOx present in the exhaust gases can be treated by the catalyst 5. In another alternative, this proportion is lower than 1, which serves to economize additional fuel consumed while treating part of the Nox present in the exhaust gases in order to conform to the standards in force.

The computer also stores an injector triggering threshold. The computer receives the real time data on the engine speed and, also in real time, calculates both the quantity of NOx present in the exhaust gases issuing from the engine and the quantity of fuel required to reach the desired stoichiometric proportion. When the ratio of the required quantity of fuel to the quantity of nitrogen oxides is lower than the triggering threshold, the computer actuates the additional injector, and when the ratio is higher than the threshold, the additional injection is not ordered. A ratio higher than the triggering threshold corresponds to a quantity of NOx issuing from the engine 1 which would require too much additional fuel to be treated on the catalyst.

According to another embodiment of the invention, the fuel-air ratio of the exhaust gases leaving the engine is not evaluated by comparing the manufacturer's data with the measurements of engine speed, but is directly measured by the fuel-air ratio probe 3. The computer 6 calculates the quantity of fuel required to reach the desired stoichiometric proportion and compares the ratio of the required quantity to the estimated quantity of NOx and triggers the additional injection if this ratio is lower than the triggering threshold. This embodiment, by combining in real time the measured data with the calculated data, serves to have a better evaluation of the required quantity to be injected. This may also help to compare the calculated forecasts with the measured quantities, in order to detect any irregularity in the sensor or computation system.

The reducer injected by the additional injector 4 is not necessarily identical to the fuel used by the internal combustion engine 1. However, the fact of using the same fuel eliminates the need for an additional tank. The internal combustion engine is preferably a diesel engine and the fuel injected by the additional injector is diesel fuel. In diesel engines, it is routine for the gas intake devices in the combustion chambers and the injection devices in the combustion chambers to be set so that the combustion chambers are saturated with oxygen. Thus, the exhaust gases from the diesel engine are generally oxidizing.

The diesel engines operating with a type of combustion called homogeneous-charge combustion have exhaust gases of which the concentration of reducing agents is high and the NOx emissions limited, so that the quantity of additional fuel to be injected remains low.

The propulsion method and system of the invention may be compatible with an internal combustion engine using gasoline, of which the exhaust gases are poor in reducing compounds. For example, in stratified-charge combustion engines, although the quantity of NOx produced by the engine has been reduced by a lower combustion temperature, the method of the invention nevertheless serves to combine the advantages of a three-way catalyst with very low fuel consumption.

The injector 4 is additional in the sense that it is specific and is added to the main injectors, not shown, of the internal combustion engine 1. It may be arranged directly at the inlet of the catalytic pot 5. The injector 4 may also be provided by the main injectors, in which case it is additional in the sense that the injection sequence concerned by the triggering means of the invention is additional in comparison with the normal injection cycle of the engine 1. This additional injection can take place at the start of the expulsion of the gases from the combustion chamber.

The particularly advantageous combination of such a propulsion system with the use of particulate filters will now be described.

According to the second embodiment shown in the figure, the second section 2 b of the exhaust line comprises, in the gas flow direction: a second fuel-air ratio probe 10, the catalyst 5, an upstream temperature and pressure sensor 11, a particulate filter 12 and a downstream temperature and pressure sensor 13. The particulate filter 12 is responsible for retaining the soot and unburnt particulates emitted by the engine 1. The particulate filter 12 may be catalyzed or not. The second fuel-air probe 10 downstream of the additional injector 4 serves to control the calculation of the quantity of the injector to reach the stoichiometric conditions during the normal running of the engine 1. The additional injector 4, the second lambda probe 10 and the temperature sensors 11 and 13 are also used for regenerating the particulate filter. An exothermic reaction is obtained in the catalyst 5 in order to heat the exhaust gases entering the particulate filter at a sufficient temperature to burn the soot and filtered particulates. With a catalyzed particulate filter, the exhaust gas temperature required to reach regeneration is lower than with an uncatalyzed particulate filter.

In the third embodiment shown in the figure, the second section 2 b of the exhaust line 2 comprises, in the gas flow direction, the upstream temperature and/or pressure sensor 11, the particulate filter 12, the downstream temperature and/or pressure sensor 13, the second fuel-air ratio probe 10 and the catalyst 5.

In the fourth embodiment shown in the figure, the second section 2 b of the exhaust line 2 comprises in the gas flow direction, the second fuel-air ratio probe 10, the option sensor 11, a catalyst 5 incorporated in the particulate filter 12 and the downstream sensor 13. The fourth embodiment has the advantage of reducing the number of components to be installed on the exhaust line 2. However, the second and third embodiments offer greater flexibility in the layout of the various components.

In each of the three embodiments containing particulate filters 12, the filter may consist of silicon carbide, cordierite, or any other ceramic or metal structure which serves to filter the particulates.

The means for determining the quantity of nitrogen oxides issuing from the engine 1 may be a computer 6 comparing the engine speed measurements in real time with a mapping of this prerecorded operation stored in the computer.

The means for determining the quantity of fuel required to reach a stoichiometric proportion may also be a computation comparing the real time measurements with a prerecorded mapping, or may be the second fuel-air ratio probe 10.

The invention, at the cost of a controlled extra consumption, serves to obtain the desired NOx treatment efficiency with a lower development cost of the propulsion system than for NOx trap type systems.

The flexibility of the method is particularly advantageous for propulsion systems used on construction sites, in which the extra consumption is not the key criterion and where the priority is to treat the nitrogen oxides completely. To adapt this propulsion system to construction site applications, it suffices to modify the triggering threshold. 

1-12. (canceled) 13: A propulsion system, comprising: an internal combustion engine; a catalyst for reducing nitrogen oxides NOx contained in the exhaust gases of the engine; a fuel injector arranged upstream of the catalyst; means for determining a quantity of nitrogen oxides NOx issuing from the engine; means for determining the quantity of fuel required for attaining a proportion lower than or substantially equal to the stoichiometric proportion with regard to the quantity of nitrogen oxides NOx; and means for triggering injection of the required quantity when the ratio of the required quantity to the quantity of nitrogen oxides NOx is lower than a triggering threshold. 14: The propulsion system as claimed in claim 13, in which the engine is a diesel internal combustion engine and the fuel injected by the additional injector is diesel fuel. 15: The propulsion system as claimed in claim 13, in which the means for determining the quantity of NOx comprises a computer configured to receive data on engine speed and on injection into the engine combustion chambers and for comparing these data with stored data. 16: The propulsion system as claimed in claim 13, in which the means for determining the quantity of NOx comprises, downstream of the engine and upstream of the injector, a device for measuring fuel-air ratio of the exhaust gases. 17: The propulsion system as claimed in claim 13, further comprising. a particulate filter upstream or downstream of the catalyst. 18: The propulsion system as claimed in claim 13, further comprising a particulate filter incorporating a catalyst. 19: The propulsion system as claimed in claim 17, in which the injector is placed upstream of the particulate filter and is actuated to regenerate the particulate filter. 20: The propulsion system as claimed in claim 17, further comprising a second device for measuring concentration of reducing agents downstream of the injector and upstream of the catalyst. 21: The propulsion system as claimed in claim 17, further comprising a temperature probe upstream or downstream of the particulate filter, and/or a pressure probe upstream or downstream of the particulate filter. 22: The use of a propulsion as claimed in claim 13, in which the means for determining the quantity of nitrogen determines the proportion of nitrogen oxide escaping from the combustion chambers of the engine. 23: The use of the propulsion system as claimed in claim 22, in which the exhaust gases upstream of the additional injector are poor in reducing agents. 24: A propulsion method, in which the exhaust gases from an internal combustion engine are treated by injecting hydrocarbons into an exhaust line upstream of a NOx reducing catalyst, comprising: determining a quantity of NOx present in the exhaust gases upstream of an injector; triggering hydrocarbon injection when a NOx quantity threshold is reached; and adjusting the quantity injected so that the quantity of reducing agents is lower than or substantially equal to the stoichiometric proportion with regard to the quantity of NOx present in the exhaust gases. 